CHAPTER 8
Snow-roosts
HUMANS EVOLVED in the tropics and retain the characteristics of tropical mammals. Our vulnerability to cold can be reduced by clothing, shelter and experience, but people still die of hypothermia in their homes during winter and some out on the hills even in summer. How grouse use snow to keep warm is interesting, and has lessons for humans.
Caribou and musk ox are so big that they lose relatively little heat from their bodies and do not need to burrow under snow for insulation, unlike many smaller animals.1 In Scotland, mountain hares make snow-burrows, and in severe frost abroad so do redpolls, goldcrests, tits and many other small birds.2 All grouse species rest and roost in snow.3 Bishop Pontoppidan of Bergen wrote in 1755 that willow ptarmigan ‘seek covering and warmth by burying themselves in the snow’,4 and Thomas Pennant in 1771 that Scottish rock ptarmigan in winter are ‘the colour of the snow, in which they bury themselves in heaps, as a protection from the rigorous air’.5 The smallest willow and rock ptarmigan live in interior regions such as Alaska and Siberia, where undrifted, deep, soft snow prevails in calm, cold winters, and we infer from this that the excellent insulation provided by such snow allows the birds to be smaller (see Chapters 3 and 4).6
RESTING ON OR IN SNOW
When red grouse or Scottish ptarmigan rest in strong wind-chill for a few minutes, they raise their feathers to form a canopy that encloses much air, tuck in their heads, and crouch, facing the wind like balls, with minimal surface area. For longer spells of daytime resting in snow, especially around midday, they take shelter by digging
bowls (Fig. 110) or sometimes by burrowing sideways to sit at the entrance.7 Red grouse occasionally use closed burrows during the daytime (see below), and blackgame and capercaillie also dig bowls and both open and closed burrows.8 Seton Gordon watched in daytime as Scottish ptarmigan moved to hollows made by his footsteps in crusted snow, and a bird sat in each hollow, dozing or pecking snow until ‘herded out’ by others.9
HOW GROUSE MAKE AND LEAVE SNOW-ROOSTS
Birds of all four British grouse species make surface hollows by scratching with their feet and shovelling with their bills. To burrow in powder snow they often fly in,10 either into a steep drift or down into horizontal snow, and quickly bury themselves without leaving marks on the surface. Frequently they walk to the site and then dig burrows, and those in a group start digging simultaneously.11 Each bird kicks snow backwards to plug the entrance, and using feet and body it tunnels forwards, finally sitting in a chamber with a snow-roof above it. Finnish blackgame sometimes burrow in a sinuous or strongly curved line, or in a complete loop, perhaps to fool predators.12
FIG 111. Ptarmigan in a snow-burrow. After flying into the snow, it burrows forwards, kicking back excavated snow to fill the entrance, and then sits in an air chamber. In the morning, it stands up to push its head through the roof, climbs through this hole, and walks or flies away. The roof is about 8cm thick and the chamber 16cm high. Key: (A) floor of compressed snow; (B) marks where bird has touched or pecked snow; (C) route of exit hole broken through at dawn; (D) loose snow kicked back to seal the entrance hole and tunnel; (E) ground below snow. (Drawn by Dave Pullan)
In the morning, birds often fly straight out through the roofs of burrows, leaving wing marks on the snow. More usually, a grouse in a burrow stands up, pushing its head through the roof. After a brief look, it climbs up through the roof, leaving the roofing snow as broken plates that lie at the edge of the exit hole or nearby (see Fig. 101). It then voids one or two caecal droppings on snow outside the hole, and walks or sometimes flies to feed. When hundreds of red grouse have roosted in burrows and walk to the nearest heather for breakfast, their tracks and droppings litter the formerly immaculate snow (see Figs 101 % 102).
Individuals use a new bowl or burrow each night, though in hard, icy snow the same bowl is occasionally used for two or even three nights by a Scottish ptarmigan, with dung piling up. Such hard snow is far less frequent on moorland, and red grouse in such conditions move downhill to roost in heather, or uphill to find softer snow.
When disturbed during daytime from burrows in powder snow, Alaskan rock ptarmigan, willow ptarmigan and white-tailed ptarmigan often fly a short distance (often just 20-30m), then dive headlong into the snow and disappear immediately.13 Staff at Scottish ski centres often see the same behaviour when their piste-grooming machines disturb red grouse from burrows at night, and occasionally in daytime during heavy snowfall.14
FIG 112. After a cold night, a pack of grouse have walked from their closed snow-burrows. (Adam Watson)
TYPES OF SNOW IN RELATION TO ROOSTING
The Inuit and Sámi have 40 separate words for different kinds of snow, but here we note only the commonest kinds. Most snow comprises hexagonal crystals whose points stick to each other in a snowflake. After a heavy fall of fluffy flakes during a calm night on moorland, snow covers all heather like a vast umbrella. The moor appears deserted in the morning, but red and black grouse are under the snow and emerge later, usually in the afternoon.15 When such a fall blankets pines and understorey vegetation, capercaillie also vanish, and are probably under the snow.16
Within a few hours of snow reaching the ground, the points of its crystals start to vanish into water vapour by sublimation. This leaves the crystals as fine grains, which later begin to bond with one another. In cold, calm air, the bonding creates loose, fragile powder snow.
Wind breaks off so many crystal points during a gale-borne snowfall that you see no snowflakes in a storm. Fallen snow blows along the surface or high up into the air in writhing snow plumes, and so is continually redistributed, while sublimation removes more points and entire crystals. When this tighter powder gathers in a hollow, its consistency resembles that of flour. If you inadvertently drop a car key or ski binding into 6ocm of it you may lose it. It was in such conditions that the worst tragedy in British mountaineering history occurred: the Feith Buidhe Disaster, in which six schoolchildren died in November 1971 on Cairn Gorm owing to unwise leadership. When caught outdoors in a winter storm, the Sámi formerly slept comfortably in the powder, with just their noses peeping out.17
Wind transforms powder snow by blowing it into a myriad of tiny grains, and continual wind then packs these grains so hard that a man or caribou can walk on the surface. Such a snow covering is typical in Inuit country, Svalbard, the Russian tundra and Scotland’s high hills. Ptarmigan can dig bowls in it, but usually choose softer, more recent drifts, whose chalky whiteness contrasts with the greyish older snow.
Thaws or rain are frequent in Scotland, and subsequent frost makes the wet snow so icy that one cannot walk safely even on flat ground without crampons. A bird cannot dig into this, but finds softer snow by flying to a different altitude.18 Further change comes with a big thaw, which turns all snow into loose ice granules called ‘spring snow’, easy to dig into but too wet for a comfortable burrow.
SNOW AS INSULATION
Soft snow provides good insulation because it consists mostly of air, as you can see by standing on it, when your boots plunge almost to the ground. Just 2.5cm of rain is equivalent to well over 30cm of fresh snow, and water has a density 20 times that of cold, newly fallen snow.
By roosting in or under snow, a bird reduces its heat loss by warming the air around it, and it also suffers no wind-chill. George West found that willow ptarmigan in winter have a ‘lower thermo-neutral’ temperature (the temperature below which a resting bird in air will lose heat) of-6.3°C, compared with 7.7°C in summer.19 Birds in winter fill their crops quickly and can spend up to about 95 per cent of the 24 hours of each day under snow. Down there, they are often well within their thermo-neutral zone, surrounded by air that is warmer than -6.3°C. West calculated that they could exist at an ambient temperature of-93°C, if they had ready access to willow or other nutritious winter food and deep powder snow for roosting.
ADVANTAGES OF SNOW-ROOSTS
On a clear night, a grouse without vegetation or snow above its body loses far more heat than one would expect from air temperature alone, because extra heat radiates to the cloud-free sky.20 For example, in an air temperature of-40°C, a bird in the open on a starlit night may lose heat at the same rate as a bird in the open on a cloudy night or under trees, in an air temperature of-56°C. This applies equally to a person at night in the open, bringing with it grave risks of hypothermia. Roosting under snow prevents this heat loss, and vegetation overhead reduces it.
When ruffed grouse in Missouri roost in cedars, this cuts the heat lost from their bodies via radiation to the night sky by 60 per cent.21 Missouri birds prefer to burrow under snow, but when they cannot find enough they favour cedars, which are trees of dense foliage. Being under snow reduces their metabolic rate by 33 per cent compared with roosting in the open, while roosts in cedars cut it by 19 per cent and roosts in deciduous trees by only 6 per cent. We think that other benefits of dense coniferous foliage would be interception of rain, snow and falling frost crystals, and shelter from wind. Also, on a calm, frosty night a bird in a tree would be in warmer air than on the ground, because of the temperature inversion that is created as cold, dense air sinks.
At Chilkat Pass, British Columbia, where packs of wintering willow ptarmigan stay under snow all night and for most of the day, Dave Mossop watched what happened when a fox stalked one of the invisible birds, presumably by scent.22 All the stalks that he watched were unsuccessful, because a different bird that happened to be nearer to the fox than the bird being stalked would flush and raise the entire pack. After many years of study in Minnesota, Gordon Gullion wrote that ruffed grouse under snow are ‘virtually immune to predation, for neither raptor nor mammalian predators have much success finding birds in these burrows’.23 One reason for this is the noisiness of snow.24 The slightest crackling or squeaking of snow caused by a dog walking on it or by a man on ski or foot alerts red grouse or Scottish ptarmigan underneath, and they fly out.25
The value of snow is manifest when birds cannot use it. Blackgame suffer heavier mortality when snow is absent, or too hard to dig into, or too shallow.26 When severe frost prevails in Minnesota on successive nights without snow or with shallow snow, ruffed grouse become thin, suffer more losses to predators, and breed poorly next summer.27
Because of a warming climate, deep snow is now less frequent and icy crusts are commoner. Suitable snow for burrows has become scarcer in Scottish woodland, where we have not seen capercaillie snow-holes since 1984. Paradoxically, capercaillie may not find it easier to keep warm in the increasingly mild Scottish winters, because wet windy weather could be worse for them retaining heat, just as it is for people without waterproof clothing. This has yet to be studied.
TOPOGRAPHY AT SNOW-ROOSTS
Although most studies of snow-roosts in grouse have involved flat or gently sloping snow, Lagopus lagopus, rock ptarmigan and white-tailed ptarmigan often choose short, steep snow banks or banked drifts. Temperatures under and in snow vary with topography.28 At an air temperature of -45°C in Siberia, the soil surface under 25cm of snow had a temperature of-30°C below coniferous trees, while the surface of bare gravel in a floodplain under the same depth of snow was a warmer -15°C. This is because gravel ridges contain much air, a good insulator, whereas moist or wet soils contain much ice, a poor insulator. A bird making a snow-burrow above gravel would therefore reduce heat loss better than a bird choosing to burrow under conifers.
In blizzards, Scottish ptarmigan often fly uphill at dusk, to roost among boulders on exposed summits where most snow blows past. On these ridges a human can barely stand and sometimes must crawl, yet the birds settle down to roost without fuss, scattering so that each has a hollow in snow on the leeward side of a boulder.29 This is the favourite site for ptarmigan in the Cairngorms and also in northeast Greenland; the drift behind the boulder is often not much bigger than the bird itself.30 There it sits facing the wind, with the top of its head 2-3cm below the snow surface, its bill making marks at the top of the snow-wall in front, and its tail touching the snow behind. The positions of these marks and of faeces show that the bird keeps a roughly similar seat all night. If the wind changes direction during the night, however, the direction of spindrift moves gradually round and the bird with it, as revealed by the more widely scattered positions of the marks and faeces next morning.31
Ptarmigan sometimes choose to roost on cliff ledges or cliff-top cornices, where they are safe from foxes or stoats. In one case, about 120 birds dug bowls in hard snow on a 40-50-degree gradient below an overhanging snow cornice above 200m-high cliffs.32 Smooth ice glazed the awesome precipices, which plunged to sunless depths far below. The birds chose well.
SNOW DEPTH FOR BOWLS AND BURROWS
In Scotland, red grouse and ptarmigan usually dig open bowls, making them deeper in hard frost, and in severe frost burrowing underneath snow.33 In hard snow that can bear a human’s weight, they dig bowls 2-3cm deep; in spring snow or slightly frozen snow they dig bowls 5-7cm deep, or occasionally up to 20cm deep; and in powder snow they dig bowls 20-30cm deep. On calm nights they occasionally burrow straight down for 20cm in powder snow and then sideways, or sideways for 20-50cm into a vertical drift. Red grouse often burrow sideways for 50cm and occasionally for up to 1m.34
Finnish blackgame usually need soft snow that is 27cm deep for burrowing, but can penetrate quite hard crusts within the snow-pack.35 Snow as shallow as 23-25cm can suffice in early winter, when the birds burrow down to the ground and hence lose less heat because the ground is warmer than the snow and air above it. Even a snow depth of 21cm can be enough, although the birds then have to accept a thinner roof. In very deep snow they usually burrow only 33cm down, and have a roofless than 10cm thick.
Cock capercaillie require at least 50cm of snow to burrow and hens 40cm, although their burrows are not usually deeper than 20cm below the surface.36 Indeed, blackgame and capercaillie avoid making burrows more than 40cm below the surface, even in snow of far greater depth. Perhaps this is because a bird in a deeper burrow would be more vulnerable to predators (see ‘Disadvantages of Snow-roosts’ below).
OPEN BOWLS OR CLOSED BURROWS
In snowy conditions, red grouse and Scottish ptarmigan choose where to spend the night at dusk. Although suitable snow often occurs near their feeding sites, sometimes it is too loose or hard to dig a bowl or burrow, or a storm may have arisen with heavy drifting. In such situations they fly off to find better conditions. In stormy weather, Scottish ptarmigan usually fly uphill to exposed summits, while red grouse search out less exposed drifts. Willow ptarmigan roost in burrows under snow only at air temperatures below -10°C,37 and burrows of red grouse in northeast Scotland are typical on calm, frosty nights.
Scottish ptarmigan dig open bowls much more frequently than burrows. Windy nights are the norm on high Scottish hills, as they are in the terrain of rock ptarmigan in Greenland and north Alaska.38 If a bird were to burrow inside
FIG 113. Entrance to (on the left, under ski stick – the hole around the stick is made by the basket) and exit from (right) a closed red grouse snow-burrow. The burrow’s ceiling has collapsed, showing the line of the entrance corridor. Other marks in the snow are footprints made by birds in the morning. (Adam Watson)
FIG 114. Two open snow-bowls used by ptarmigan on the Cairnwell. The holes are in a small drift of powder lying on the icy grey snow seen in the foreground. The marks on the drift show where a bird tried several times to climb its steep side before succeeding. (Stuart Rae)
a deep drift at a sheltered site, it would run a serious risk of being entombed by metres of drifted snow during a blizzard, whereas a bird in an open bowl at an exposed site is completely safeguarded from this hazard. By contrast, in central Alaska, where deep powder snow and calm air are typical, rock ptarmigan usually burrow.
On summer nights, when snow is too loose and wet for a burrow, Scottish ptarmigan often dig snow-bowls, even using tiny remnant snow patches on warm August nights. In such cases, reducing heat loss seems irrelevant, so perhaps the birds are attempting to reduce the risk from predators or just trying to keep cool.
On nights when air temperatures are above 0°C, blackgame roost in trees.39 At air temperatures below -3°C, they usually make closed snow-burrows, although they must use trees if the snow is too hard to penetrate. They dig open bowls about 11-19cm deep in soft snow that is too shallow for burrows, and in crusty or granular snow that would be unsuitable for a closed burrow. Occasionally they roost in burrows even at air temperatures above -3°C, which raises the idea that they may be attempting to reduce predation rather than heat loss.40
CONDITIONS IN A SNOW-BURROW
Finnish blackgame make snow-burrows that consist of a chamber 16-31cm high (usually 18-24cm), with a snow-roof 1-20cm thick (usually less than 10cm).41 Because still air provides good insulation, with a heat conductivity only a fifth that of snow, a spacious chamber avoids loss of heat through conduction because the bird’s body does not touch the sides. Air temperature has been measured inside a greyhen’s roost chamber at 5mm above her back (Table 18), and reached 12°C for most of the night.
All four British grouse species in burrows at typical depths fairly near the surface can readily detect an approaching person on foot, on skis or on a machine, perhaps by vibration through the snow. Arto Marjakangas inferred that a bird burrowing more deeply would not hear approaching predators, because a tape recorder buried at a depth of 20cm in the snow detected hardly any audible sound from an alarm clock on the surface.42
In northeast Siberia, Alexander Andreev used tame willow ptarmigan for his studies on winter adaptation.43 He wrote, ‘It took at least two nights to acquaint the laboratory-raised ptarmigan with snow and train them to use snow as
# Temperature taken 15cm above the snow surface in the case of ruffed grouse, in others not specified.
* Reached 2°C after 60 minutes and 12°C in 90 minutes. The temperature of undisturbed snow at the level of the burrow was -4°C.
** After six hours.
protection from frost. To do this, I first prepared hand-made snow burrows with an open roof and tunnel. After placing the birds inside the burrow, the tunnel and roof were gradually closed with pieces of packed snow.’ Later the birds made their own burrows. The downward flow of heat through the floor of a roost chamber exceeds that going through the roof 1.6-fold, but the willow ptarmigan’s densely feathered snowshoes prevent its body heat from melting the snow on the floor.44 Such melting would reduce insulation by removing air pockets and also by increasing conduction of heat via the melt-water.
Andreev also used tame birds to study how hazel grouse, a very small species, cope with cold.45 After exposure to cold for 30 minutes inside a cage on undisturbed snow, a cock buried himself. The temperature in his roost chamber varied from -15°C up to 0.3°C, depending little on the outside air temperature. How the cock buried himself had far more influence, including factors such as the tunnel’s length, bends in the tunnel, and especially whether he left an opening between the chamber and the entry tunnel, or closed it with a plug of snow. If he made a tight plug, the temperature in the chamber could approach 0°C even in very severe frost, whereas it fell to -7°C or -9°C if he left an opening. At outside air temperatures of-40°C to -45°C, the chamber’s temperature (see Table 19) came close to 0°C occasionally. When the outside temperature rose above -35°C, the cock avoided becoming overheated by making an opening in the roof with his bill, whereupon the chamber temperature fell quickly from -5°C to -16°C.
Notes
For capercaillie the figure covers the dark part of midwinter, for hazel grouse from November to March.
In Leningrad and Novgorod regions, at air temperatures of -20°C down to -38°C, willow ptarmigan, black grouse, capercaillie and hazel grouse spent 22-23 hours in burrows and fed for one to two hours in nearby trees (Potapov, 1974). Potapov’s Table 2 shows willow ptarmigan, rock ptarmigan, blackcock, cock capercaillie and hazel grouse spending 17, 20, 22, 19 and 21 hours respectively in burrows at an outside air temperature of -20°C, the temperature inside ranging from -10°C to -6°C. The time spent outside varies with air temperature, with about two hours spent outside in very cold air (from -50°C to -40°C), but four hours out at -20°C, and six to ten hours out at from -10°C to -5°C.
# Blackcocks eating entirely oats at a feeding station, feeding once daily in the morning.
PROPORTION OF THE DAY SPENT IN SNOW-ROOSTS
At midwinter in colder parts of their range, grouse species spend most of the 24 hours of each day under the snow (see Table 19), emerging only to feed or if disturbed. At Chilkat Pass, British Columbia, where diurnal raptors kill many willow ptarmigan, birds spend the daylight hours out of sight under snow, emerging for intense feeding during twilight at dawn and dusk.46 They do not eat in early afternoon, the warmest time of day, perhaps to reduce the risk of being killed. East Greenland lies so far north that the sun is below the horizon in December and January, resulting in far shorter day length than at Chilkat Pass, and rock ptarmigan in December feed hurriedly during the two hours of faint light around noon.47
DISADVANTAGES OF SNOW-ROOSTS
After a bird goes to roost in a snow-hole when the outside air temperature is below -3°C, warmer air sometimes arrives during the night and melts the snow surface. If a frost then ensues before morning, a crust of ice forms on the surface, potentially trapping birds. The gravest risk would be when heavy rain falls and a sharp frost follows, creating a thick, hard, icy crust that may be impenetrable. Cases have been reported of blackgame unable to get out of their snow-burrows, notably in Finland in 1983, when skiers at one place released 40 birds, many of them exhausted and bleeding after trying to struggle through a hard crust that had formed overnight.48 Arthur Bent related an account of willow ptarmigan in Newfoundland frequently being imprisoned in snow and later found dead in spring.49
Another hazard is related to predators. When a fox approaches, capercaillie tend to stay in their burrows until the last possible moment and then burst forth,50 so when ice slows them, they can be easy prey.51 Out of 278 winter deaths of rock ptarmigan that Bob Weeden found at Eagle Creek, predators killed most of them, although eight had died in a small snow slide and he judged that two had probably been trapped in snow-roosts during storms.52 We have not seen such cases in Scotland.
EFFICIENCY OF GROUSE VERSUS HUMANS IN MAKING A SNOW-SHELTER
An experienced Inuit with a snow knife takes about an hour to build an igloo, the time varying with the snow conditions and the size of the igloo, and two will finish the job sooner.53 An experienced Scottish party with snow shovels takes one-and-a-half to three hours to make a comfortable snow-hole.54
In contrast, Lagopus lagopus and rock ptarmigan make bowls or burrows within 10-30 seconds, depending on the hardness of the snow. Birds of Lagopus species often dive headlong into loose powder, taking less than a second to vanish. When they burrow into more cohesive snow they seal the entrance with excavated snow in two to three seconds.
SUMMARY
Grouse use snow as insulation against the cold when resting and roosting, by digging bowls where they sit with their backs open to the sky, or by burrowing underneath the surface and sealing the entrance by kicking back excavated snow. Red grouse make bowls on nights of light frost, and burrows in hard frost. Rock ptarmigan in windy climates such as Scotland, Greenland and north Alaska usually dig open bowls in wind-packed snow behind rocks, whereas in central Alaska they burrow in the undrifted soft snow that prevails there.